Looks like JimB has got it sorted. I think the most important point is that it is necessary to read the manufacturers handbook to find out what has been standardised. I know from my time with German made telephony transmission level measurement equipment, that 0dBm was 0.775 volt. In that industry, the original work was done with balanced pair cable with a standardised characteristic impedance of 600 ohm. With the progress of time and the introduction of coaxial cable systems with a characteristic impedance of 75 ohm, the 0 dBm level was still 0.775 volt into 75 ohm. This was using Siemens and W & G gear.